The present invention relates to a lithium secondary battery; and, more particularly, the invention relates to a negative electrode for a lithium secondary battery, which has large values of discharging capacity, output power density and charging-discharging velocity, and superior cycle characteristics. The lithium secondary battery can be utilized as a driving power source for electric vehicles, for memory back up, for a portable apparatus, and the like. For instance, lithium secondary batteries are assembled into electronic apparatus, such as note type personal computers, word processors, portable telephones, cordless telephones, portable facsimile machines, portable printers, headphone stereos, video movies, liquid crystal TV sets, handy cleaners, portable CD players, electric shavers, electronic translators, automobile telephones, transceivers, electric tools, memory cards, and the like.
The lithium secondary batteries can also be used as power sources for consumer goods, such as medical apparatus, for instance, pace makers, hearing aids, massagers, and the like. Furthermore, the lithium secondary batteries can be utilized as the power sources of apparatus designed for use in outer space. The lithium secondary batteries can be used as power sources by being combined with solar cells.
As the negative electrode for a lithium secondary battery, lithium metal (Li) and alloys such as Li--Al, Li--Pb, and the like have been used. However, the above conventional battery has defects, such as the tendency to cause a short circuit between the negative electrode and the positive electrode of the battery by precipitation of a resin-like lithium, and the conventional battery also has defects, such as a short cycle life and a low energy density. Currently, in order to overcome the above-mentioned defects, research on using carbon for the negative electrode is being actively pursued. For instance, some carbon negative electrodes have been disclosed in JP-A-5-299073 (1993), JP-A-2121258 (1990), JP-A-6-349482 (1994), and JP-A-7-335263 (1995). The composition disclosed in JP-A-5-299073 (1993) uses a carbon complex body comprising high crystalline carbon particles forming a core, the surfaces of which are coated with a film including metallic elements of VIII group, and the film is further coated with carbon, as an electrode material. Thereby, the carbon material having a random layer structure enhances intercalation of lithium, and concurrently, the large surface area of the electrode significantly improves the charging-discharging capacity and the charging-discharging velocity. In accordance with JP-A-2-121258 (1990), a long charging-discharging cycle life and preferable charging-discharging characteristics with a large current can be obtained by using a mixture of carbon material having hexagonal crystals, of which H/C&lt;0.15, spacing &gt;3.37 .ANG., and Lc&lt;150 .ANG., where, Lc is a size of crystal lattice in a C-axis direction, and a metal which can form an alloy with lithium. In accordance with JP-A-6-349 482 (1994), a carbon complex body, wherein copper oxide is deposited onto all or a part of the surface of graphite particles, which allows intercalation-deintercalation of lithium, is used as an electrode material. Thereby, increasing the capacity becomes possible, because a complex oxide of lithium and copper is formed reversibly onto the copper oxide, which is electrochemically reduced. In accordance with JP-A-7-335 263 (1995), the contact resistance between active materials can be reduced by adding a metal, as a conductivity assistant agent, to the carbon for use as an active material in the negative electrode or the positive electrode. The contact resistance between a collector and the active material can also be reduced by the addition of the metal to the carbon, as mentioned above, and accordingly, a decrease in the capacity can be suppressed as much as possible, even with a high rate of discharging (a large current discharging). Regarding only the negative electrode, an orientation of carbon can be prevented by adding an alloy, such as stainless steel, permalloy, and the like, in addition to a metallic carrier, such as nickel, copper, silver, aluminum, and the like, to the carbon. As a result, a large current discharging becomes possible, because a fraction of the carbon particles, the side plane of which is oriented to face the electrolyte, is increased, and a diffusion of the ions is facilitated. The inventors of the present invention have proposed a lithium secondary battery having an increased capacity, an increased power density, and superior cycle endurance characteristics, in JP-A-8-273702 (1996), wherein carbon particles bearing particles made of a metal, which can form an alloy with lithium, of utmost 100 .ANG. in diameter at their surfaces, are used as the negative electrode material. However, in any case, the power density was not sufficiently increased, because preparation of the negative electrode was difficult, and the theoretical capacity of the carbon was not utilized. Particularly, an issue such as a remarkable improvement in an aspect of the rapid charging-discharging (a large current charging-discharging) remained to be solved. Accordingly, the lithium secondary battery was insufficient in energy density and power density for use on electrical vehicles and motorcycles.
As mentioned above, when the carbon material or the carbon complex material was used as a material for the negative electrode, problems remained, such as the theoretical capacity of the carbon not being utilized, the preparation of the carbon material for the negative electrode being difficult, and the fast charging-discharging (a large current charging-discharging) being impossible, and so these problems remained to be solved.